1. Field of the Invention
The present invention relates to optical waveguides, especially glass optical fibers, having a non-circular high refractive index region which is bounded by a lower refractive index region, where the boundary between both regions comprises flat regions with well-defined small radius edges.
2. Invention Disclosure Statement
Fiber lasers are usually based on glass fibers which are doped with laser-active rare earth ions. These ions absorb pump light, typically at a shorter wavelength than the laser or amplifier wavelength, which excites them into some metastable levels. This allows for light amplification via stimulated emission. Such fibers are often called active fibers. They are gain media with a particularly high gain efficiency, resulting mainly from the strong optical confinement in the fiber's waveguide structure. Most laser fibers are fabricated by preform-based methods. Most of today's structures involving a preform are realized by modified chemical vapor deposition (MCVD) outside vapor deposition (OVD), Vapor phase axial deposition (VAD), Plasma-activated chemical vapor deposition (PCVD), plasma outside deposition (POD) and direct nanoparticles deposition methods (DND).
In MCVD, a mixture of oxygen, silicon tetrachloride (SiCl4) and other substances such as germanium tetrachloride (GeCl4) is passed through a rotating silica glass tube, which is heated from outside to approximately 1600° C. Chemical reactions in the gas form a fine soot of silica which coats the inner surface of the glass tube near the burner and is sintered into a clear glass layer. The burner is continuously moved back and forth along the tube. Towards the end of the process, the gas mixture is modified to form a layer with higher refractive index, the precursor of the fiber core. Finally, the tube is collapsed by heating it to about 2000° C. and evacuating the interior of the tube such that the outside atmospheric pressure will cause the tube to collapse. There are some drawbacks with the solution doping process: the salts have a tendency to self-associate into chemical structures and thus form easily high local concentration variations into the glass. The porosity of the soot layer is also difficult to control, and subsequently the dopant concentrations. MCVD with solution doping is a multi-step, iterative process yielding a doped core of 2-10 layers. The low number of core layers limits the accuracy and flexibility of doping and refractive index profiles. Also the throughput time is fairly large due to multi-step process and this makes the fiber development work slow and expensive.
OVD is a process where the silica soot is deposited on the surface of some target rod, such as a glass mandrel. Together with the material precursors such as SiCl4, a fuel gas such as hydrogen or methane is supplied to a burner which is again moved along the rotating rod. After the deposition, the target rod is removed, and the preform is consolidated in a furnace, where it is also purged with a drying gas for lowering the hydroxyl content. The atmospheric conditions and the type of burner required yield relatively high temperatures. Therefore, any non circular structure that will be coated by such a technique will become deformed, i.e. the edges will become rounded or even the whole structure will become nearly circular once again.
Vapor phase axial deposition (VAD) is similar to OVD, but uses a modified geometry, where the deposition occurs at the end of the target rod. The rod is continuously pulled away from the burner, and very long preforms can be made. Consolidation of the material can be done in a separate zone melting process. An important difference to OVD and MCVD is that the doping profile is determined only by the burner geometry, rather than by a variation of the gas mixture over time.
Plasma-activated chemical vapor deposition (PCVD) uses deposition inside a tube, similar to MCVD, but significantly different in that soot deposition on the inner wall does not occur; and the power source and temperature are significantly different too. U.S. Pat. No. 6,138,478 by Neuberger et. al. e.g. discloses a device and method for uniform plasma chemical vapor deposition of silica/doped silica onto an elongate substrate to form an optical fiber preform Microwaves instead of a burner are used for heating the deposition region. Initial deposition is slow, but very precise. Furthermore, in contrast to some other techniques, PCVD will result in a clear glass layer. No thermal annealing, sintering, vitrification or similar procedures are required. As the PCVD procedure takes place at low pressure and at a relatively low temperature level, the doping concentration can be varied in a large range of values. When fluorine is the dopant and a pure silica core is the substrate, larger numerical apertures than with most other techniques is achievable.
The preforms for multimode fibers, particularly for large core fibers, are often fabricated using plasma outside deposition (POD), where an outer fluorine-doped layer with depressed refractive index, later forming the fiber cladding, is made with a plasma torch. The core can then be made of pure silica, without any dopant. This procedure uses high temperatures at atmospheric pressure. Thus the non-circular substrate might become deformed.
DND as described by Tamela et. al in proceedings of SPIE Photonics West 2006, Vol. 6116-16 (2006), Integrated Optoelectronical Devices: Optical Components and Materials III, is another process based on the combustion of gaseous and atomized liquid raw materials in an atmospheric oxy-hydrogen flame. Rapid quenching and a short residence time produce a narrow particle-size distribution. The DND burner ensures the proper provision of materials into the flame. The DND process can be described as a special form of outside soot deposition where nanosize particles of dopants are inserted into the target simultaneously with silica deposition. The glass formation and doping stage is followed by sintering, which then results in a solid glass preform. This process is thus a modified nanoscale approach to the older OVD process. The doping and glass formation is done in one step using a DND burner developed for this purpose. The core-index-raising and laser-active rare earth elements are fed in the process in liquid phase directly into the reaction zone. A SiCl4 gas bubbler is used as a source for the silica base of the fiber preform. The glass particles are doped as they form in a fast reaction. After the deposition phase, the alumina mandrel is gently removed from the grown perform and handling tubes are attached to it. The preform is then inserted into a furnace where the first step is drying and cleaning. Finally, the porous glass is sintered into a solid clear core preform. Typical DND preforms are large in diameter and relatively short. This may be useful in making fibers with rectangular or other non-circular cores and claddings, multi-core fibers, or coupled multiple-waveguide fibers. Sintering stage with this method needed to form a solid glass preforms still requires extremely high temperatures.
The temperatures, established in mentioned procedures, easily exceed 1500° C. They can even be as high as 1700° C. and thus close to the melting temperature of (fused) silica, typically the best choice for the core material.
Drawing optical fibers from preforms also presents problems to providing precisely-shaped core fibers, as it commonly uses high temperature ovens operating at temperatures of 1900-2000° C. to draw the actual optical fiber, although the actual temperature of the glass within the drawing ‘oven’ may be significantly lower.
This presents problems in maintaining preforms with noncircular core shapes, and therefore in obtaining optical waveguides (or fibers) with precisely designed edged cores. Applications with active optical fibers, especially such as fiber lasers, benefit from non-circular pump cores (first cladding) and need edges between a pump core and its cladding which are sharper to obtain higher absorption efficiencies.
Waveguides, especially glass optical fibers with non-circular core regions have proven their use for several years for active fibers. Due to their mode-mixing abilities, core geometries like D-shape, square and octagon shapes are advantageous as pump cores in laser active fibers or for planar waveguides as mode-scrambling cores in beam homogenizing devices to obtain so-called flat-top beam profiles. Such waveguides were not drawn optical fibers.
Hayes et al (“Square core jacketed air-clad fiber” in 30 Oct. 2006/Vol. 14, No. 22/OPTICS EXPRESS 10345”, propose silica micro structured fiber technology, in which light is guided via the inclusion of microscopic air tubes for multi-mode, large-core fiber designs. For example, a single ring of air tubes can be used to optically isolate a large fiber core and create a waveguide from a single material. These structures are typically referred to as jacketed air-clad (JAC) fibers. JAC fibers with non-circular core geometry can be tailor made to a specific core size and NA requirement and that sharply defined vertices in the core shape can be preserved throughout the fiber drawing process. They report the fabrication of a JAC fiber with a large square core and show that the near field in this fiber has a top-hat intensity profile. The square core was formed by stacking together many circular silica rods of two different sizes. The advantage of constructing the core region from stacked elements is that it offers enormous design flexibility in terms of the core shape and composition.
In U.S. Pat. No. 4,106,847 by Arnaud, an optical fiber waveguide is disclosed in which cladding surrounds a fiber core having an elliptical fiber core. Optical fibers may thus be less sensitive to bending losses than fibers with circularly symmetric profiles if bending takes place in one preferred plane. However, elliptical shapes are limited for example in their polarization maintaining behavior. In applications such in welding, fibers with a higher symmetry yield a better performance. For some applications the polarisation maintaining behaviour might even be disadvantageous and thus undesired. Welding is such an application. Fibers with a higher symmetry as in the current invention (like square-shaped or octagonal-shaped fiber) can yield a better performance as no polarisation is maintained.
Typical of a group of patents dealing with the enhancement of power development for fiber lasers, by increased efficiency of transferring pump power to the active fiber laser core, selected for this aspect of the background, U.S. Pat. No. 6,157,763 by Grubb et al, U.S. Pat. No. 5,949,941 by DiGiovani, U.S. Pat. No. 5,291,511 by Hanna, and more recently U.S. Pat. No. 6,959,022 by Sandrock et al. present cladding-pumped optical fibers that include a core that includes an active material, a multimode inner cladding, generally called a pump core, disposed about the active core, and a ‘second’ cladding disposed about the multimode pump core. In many cases, the second cladding is a polymer which is applied as the optical fiber is being drawn. The inner cladding has different shapes, such as rectangular, D-shape, hexagonal, or star-shaped. All of which shapes are hard to establish and maintain precise edges during preform fabrication, if a second cladding is employed, and in all cases especially during standard optical fiber drawing. To reduce problems which might arise from polarizing effects, Sandrock et al. note that the non-circular pump cores should have some symmetry about planes drawn through the fiber center. This aspect can be handled within the new invention described below.
The present invention provides a solution to problems in obtaining and maintaining the preciseness of the edges in non-circular, non elliptical core preforms and drawn optical fibers, as desired and required for various application-specific needs.